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A G Redfield

Publications and source records attributed to A G Redfield.

At least 55 records · Page 3Linked to original sources
PubMed1981

Nuclear magnetic resonance and nuclear Overhauser effect study of yeast phenylalanine transfer ribonucleic acid imino protons.

Results directed primarily toward spectral assignment and nuclear spin dynamics are described for yeast tRNAPhe in 0.1 M NaCl, pH 7. Magnesium titrations were performed. Changes in the spectrum occur for Mg2+/tRNA ratios of about 2 and above 10. Difference spectroscopy between 43 and 29 degrees C in zero Mg2+ concentration, together with prior identification of the GU4 acceptor stem base pair, indicates early acceptor melting and is used to identify acceptor resonances. Transport of spin energy (spin diffusion) is described in tRNA together with a summary of relevant experiments. A survey of nuclear Overhauser effects (NOE's) between imino and aromatic and amino protons is included, together with some recent conclusions based on methyl NOE's and experiments with tRNAs deuterated at the purine C8 position. Assignment of the imino NMR spectrum on the basis of these and previous data is reviewed and discussed in detail. Preliminary distance estimates based on the NOE for AU and GU4 base pairs are in reasonable agreement with the expected distances.

Amines↗
PubMed1980

Nuclear Overhauser effect in specifically deuterated macromolecules: NMR assay for unusual base pairing in transfer RNA.

We demonstrate a fairly general method for identification of NMR absorption lines of macromolecues extracted from microorganisms, based on nuclear Overhauser effects (NOE). Several NOE in tRNA are observable between resolved imino proton resonances and ring carbon resonances that are either C(2) protons of adenine or C(8) protons of adenine or guanine. Yeast tRNAPhe was deuterated at the purine C(8) positins by heating in 2H2O and also biosynthetically. NOE between imino protons and adenine C(2) protons of standard A . U base pairs would not be affected by such a label, but some other NOE that might be otherwise similar, such as those of reverse Hoogsteen base pairs, should disappear. Six NOE were shown to be from standard A . U pairs by their nondisappearance. Four NOE from methyl resonances to aromatic proton resonances did disappear. The results disagree with previous assignments based on ring-current theories of imino proton NMR shifts.

Hydrogen Bonding↗
PubMed1979

Real-time solvent exchange studies of the imino and amino protons of yeast phenylalanine transfer RNA by Fourier transform NMR.

Real-time solvent exchange measurements using Fourier transform NMR at 270 MHz are presented. By means of the fast gel filtration column techniques originally developed for tritium exchange experiments, we were able to replace the solvent of a tRNA sample from an 1H2O to an 2H2O buffer and obtain a useful spectrum in 2-5 min. At 15 degrees C, there are 5 +/- 1 lowfield (-11 to -15 ppm relative to 2,2-dimethyl-2-silapentane-5-sulfonate) imino protons with exchange half times of minutes to hours. In addition, the m7G-46 C(8) proton and several amino protons are observed to exchange with similar rates. Analogous studies on unfractionated yeast tRNA suggest that such a class of slowly exchanging imino protons is present in several tRNAs, and that the activation energy for exchange is small [[approximatley 5 kcal/mol (21 kJ/mol)]. We speculate that these imino resonances arise from D-stem protons and that their slow exchange reflects stabilization by the numerous tertiary interactions involving this stem and the Mg2+ bound at the P-10 bend.

Base Composition↗
PubMed1978

Pulsed FT-NMR double resonance studies of yeast tRNAPhe: specific nuclear Overhauser effects and reinterpretation of low temperature relaxation data.

Cross-relaxation effects are demonstrated between the imino protons and other protons in yeast tRNAPhe and H2O. A detailed examination has been made of the observed relaxation rate of the proton resonance at 11.8 ppm from DSS as a function of the D2O content in the solvent. This result, as well as the size and number of observed nuclear Overhauser effects, suggests that dipolar magnetization transfer between solvent H2O, amino, imino, and other tRNA protons may dominate the relaxation processes of the imino protons at low temperature. At higher temperatures the observed relaxation rate is dominated by chemical exchange. The selective nuclear Overhauser effects are shown to be an important aid in resonance assignments. By these means we were able to identify tow protons from the wobble base pair GU4 at 11.8 ppm and 10.4 ppm.

Deuterium↗
PubMed1977

An NMR study of the exchange rates for protons involved in the secondary and tertiary structure of yeast tRNA Phe.

Solvent exchange rates of all the protons of yeast tRNAphe resonating in the lowfield NMR region (-11 to-15 ppm from DSS) have been measured by saturation-recovery long-pulse Fourier transform NMR. All these protons in yeast tRNAphe are in the fast exchange limit with H2O relative to their intrinsic longitudinal relaxation processes. Most rates show very little temperature dependence; however, tertiary base pair protons are preferentially destabilized in the absence of Mg++ at higher temperatures. The measured exchange rates are between 2 and 125 sec-1 for a temperature range from 10 degrees C to 45 degrees C and MgCl2 concentrations between 0 and 15 mM.

Anticodon↗
PubMed1976

NMR study of relative oxygen binding to the alpha and beta subunits of human adult hemoglobin.

NMR spectra of the downfield region of normal adult hemoglobin are reported as a function of oxygenation and temperature. Spectra were run in D2O at pD 7.4. A specially made NMR tube insert allowed precise measurement of the degree of oxygenation and of methemoglobin formation before and after taking the NMR spectrum. Plots of the estimated intensity of the most downfield prominent NMR peak, identified as arising from a deoxy-beta subunit by Davis et al. ((1971) J. Mol. Biol. 60, 101-111), versus the average degree of oxygenation y, measured optically, yield a nearly straight line within experimental error, for samples stripped of organic phosphates and for samples containing 2,3-diphosphoglycerate or inositol hexaphosphate. Intensities of peaks further upfield than this peak, previously attributed to deoxy-alpha subunits, are difficult to measure directly especially for samples containing inositol hexaphosphate. The latter samples show broadening in these alpha peaks as the degree of oxygenation increases. This extra broadening appears to increase with temperature. Linearity of the beta peak intensity with oxygenation is expected if there is no large oxygen affinity difference between alpha and beta subunits. However, the cooperativity of binding, and inaccuracy of the data, make it impossible to make accurate estimates of affinity differences.

Adult↗
PubMed1975

Proton magnetic resonance studies of alpha-keto acids.

Pulsed Fourier transform proton magnetic resonance was used to study alpha-ketoglutaramic, and several other alpha-keto acids in aqueous solutions as a function of pH. Most alpha-keto acids were found to exist in equilibrium with the hydrate (gem-doil). The equilibrium position favors the nonhydrated alphs-keto acid at neutral pH, but at low pH values (below the pKa of the alpha-carboxylic acid group) the hydrate predominates. We found evidence that alpha-ketoglutaric acid exists in a third equilibrium form which is assigned to the lactol. alpha-Ketoglutaramic acid (the alpha-keto acid analog of glutamine) which is known to exist predominantly in a cyclic form at pH 7.0 was shown to exist as a cyclic structure over a wide pH range. However, the cyclic form is an equilibrium mixture of 2-pyrrolidone-5-hydroxy-5-carboxylic and 1-pyrrolin-2-one-5-carboxylic acids.

Deuterium↗